Electrical components emit electromagnetic radiation. The Federal Communications Commission (FCC) regulates the amount of electromagnetic radiation that can be emitted form various classes of electrical and electronic devices by promulgating regulations that dictate the maximum amount of electromagnetic radiation that may be emitted form different types of electrical and electronic devices. With respect to computer systems and data storage systems, manufacturers typically control the amount of electromagnetic radiation emissions by suing conductive enclosures and gaskets to prevent electromagnetic radiation from being emitted from the assemblies. Conductive gaskets and conductive enclosures reflect radiation impinging on them and thereby prevent the radiation from escaping from the assemblies into the outside environment.
Conductive gaskets and enclosures generally are superior at controlling emissions as long as contact between conductive parts of the enclosure is continuous and the interfaces between parts of the enclosure are of low impedance. If contact between parts is not continuous, or if the interfaces between conductive parts are of high impedance, the effectiveness of the conductive enclosure or gasket as a shield will be greatly diminished. Therefore, the efficiency of the shield may be adversely affected by surface conditions and/or by inadequate contact pressure between parts is not maintained at all times.
One problem associated with using these types of conductive shields is that it is difficult to ensure that adequate contact pressure is maintained between parts due to mechanical tolerance variations. Also, aging of materials used to maintain contact between parts, such as, for example, polymers which function as springs to force conductive parts into contact, often results in adequate contact being lost over time. Therefore, it is difficult, in many cases, to ensure that adequate contact will be maintained over time, and therefore difficult to ensure the effectiveness of the conductive shielding over time.
Also, ensuring adequate contact between conductive parts of enclosures of electrical components often significantly increases the overall size of the assembly comprising the subassemblies since additional structure or treatment processes (e.g., flanges, collars, surface treatment, etc.) is required in order to ensure contact between the enclosures. Furthermore, additional structure and/or surface treatment generally translates into additional costs. The requirement that surface conditions not prevent adequate contact between parts also affects the appearance of the system assembly and/or subassemblies. For example, in many cases, the surfaces must be left exposed and cannot be painted. Furthermore, the implementation of subassemblies provided by different vendors (e.g., power supplies provided by different vendors) is limited by the requirement that the surface conditions of the various subassemblies provide adequate contact. Slight variations in surface characteristics make it difficult to achieve effective shielding.
Another problem associated with conductive shielding is that, with high frequencies, it is extremely difficult to ensure that contact between parts is adequate to control emissions. Therefore, as frequencies increase, conductive shields become less effective and other methods must be implemented to control electromagnetic radiation emissions.
It is generally known that lossy materials can be used to control electromagnetic radiation emissions. Lossy materials are materials which absorb and attenuate, and only partially reflect, electromagnetic radiation energy. The ratio of absorption-to-reflecting is determined by the electromagnetic radiation properties of the material and by the electromagnetic properties of the radiation impinging on the lossy material. However, it is not known to use lossy materials on the outside of enclosures, or housings, and between subassembly housings and a chassis on which the subassembly housings are mounted, to provide an EMC solution. With respect to these types of system assemblies, conductive gaskets and materials have been used as an EMC solution for a variety of reasons.
Low RF frequencies can be effectively dealt with using conductive gaskets and materials because conductive contact between parts does not have to be as continuous in order to ensure effective shielding, i.e., larger gaps can exist between parts while still maintaining effective shielding. Also, with these types of system assemblies, conductive shielding has been used not only for electromagnetic compatibility, commonly referred to as EMC, but also to ground various subassembly housings in order to eliminate residual voltages which may exist between parts of the chassis of the system assembly. Thus, implementing conductive shielding has been viewed as being advantageous since it performs both of these functions.
Currently computer system assemblies and data storage system assemblies implement EMC solutions which utilize conductive enclosures and conductive gaskets. As computer systems and data storage systems are developed which operate at exceedingly higher frequencies, with significant emissions at frequencies larger than approximately 1 GHz, a need will exist for an EMC solution which is effective in controlling emissions at higher frequencies and which does not diminish ineffectiveness over time due to aging or with increasing frequency. At higher frequencies, conductive shielding is less effective because continuity of contact between conductive subassemblies must be maintained in order to ensure effective shielding, which, as discussed above, is very difficult in many cases, especially above 1 GHz.
In order to meet this need for an EMC solution which is effective for controlling electromagnetic radiation emissions in system assemblies at higher frequencies, the present invention provides and EMC solution which utilizes lossy materials, or a combination of lossy and conductive materials, within an electrical system assembly, such as a computer system assembly or a data storage system assembly, on the exterior of subassembly housing and/or on the chassis of the assembly.
The EMC solution of the present invention controls the emission of electromagnetic radiation from the system assembly enclosure into the outside environment as well as the emission of electromagnetic radiation within the system assembly enclosure from subassemblies, or modules, of the system which may interfere (i.e., EMI) with the operation of other subassemblies or modules of the system. Furthermore, the EMC solution of the present invention is relatively inexpensive and overcomes problems associated with surface conditions required for ensuring contact between subassembly enclosures and the overall system enclosure.